Method of microfabrication

ABSTRACT

A temperature compensated optical isolator and a method of manufacturing optical assemblies. The isolator utilizes a bimetallic element to rotate a polarization element of the isolator in response to temperature variations. The isolator maintains an effectively constant isolation over a substantially wide temperature range. Advantageously, the isolator is simple, compact and a viable solution to a wide range of optical applications. The manufacturing method employs a lamination procedure to create an array of optics-receiving micro-fixtures. The method is well adapted for the automated manufacturing of optical assemblies. Desirably, the method provides for high speed, high volume production, thereby advantageously, maintaining low manufacturing costs.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/073,900, filed Feb. 6, 1998.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to optical devices and, inparticular, to a temperature compensator for a Faraday rotator and alamination-based method for manufacturing of optical assemblies andother small components.

2. Background of the Related Art

When light rays emitted from a light source are transmitted through anoptical system, part of the light rays will be reflected at the end faceof the optical system and transmitted back to the light source, unlessmeans are employed to prevent such back reflection. For instance, intransmitting an optical signal through an optical fiber, if a light beamemitted from a laser light source is projected onto the end face of theoptical fiber through, for example, a lens, the majority of the lightthereof will be transmitted through the optical fiber as transmittedlight beam. But, a part of the light thereof will be surface reflectedat the end faces of the lens and the optical fiber and transmitted backto the laser light source. This back reflected light will again bereflected at the surface of the laser light source, thereby creatingundesirable reflection-induced noise.

To eliminate such noise, an optical isolator, as described for instancein Bellcore's Special Report, Optical Isolators: Reliability Issues,SR-NWT-002855, Issue 1, December 1993, Pages 1-3, incorporated herein byreference, may be used. This is an example of an optical device thatallows light to propagate (with relatively low loss) in one directionbut isolates reflected light from propagating in the reverse direction.Optical isolators are used to improve the performance of many devicessuch as external modulators, distributed feedback lasers, Fabry-Perotlasers, semiconductor amplifiers, and diode-pumped solid-state lasersamong others.

Optical isolators are typically passive, non-reciprocal optical devicesbased on the Faraday effect. In 1842, Michael Faraday discovered thatthe plane of polarized light rotates while transmitting through glasswhich is contained in a magnetic field. The Faraday effect isnon-reciprocal, meaning that the direction of rotation is independent ofthe direction of light propagation, and only dependent upon thedirection of the magnetic field. Most commercial optical isolatorsutilize this effect to isolate various parts of an optical communicationsystem from reflection-induced noise.

Typically, an optical isolator consists of a magneto-optical materialcalled a Faraday rotator which is sandwiched between a pair ofpolarization elements commonly referred to as a polarizer and ananalyzer. The Faraday rotator is used in optical devices, such as theoptical isolator, to rotate the plane of polarization that is incidentupon it by a predetermined amount, usually by 45° either clockwise orcounter clockwise. Typically, the Faraday rotator is a garnetcrystalline structure with an inherent magnetic field, so that thedirection of Faraday rotation is predetermined. In some cases anexternal magnetic field may be needed to activate the Faraday rotator.In such cases, the direction of Faraday rotation is dependent on theorientation of the magnetic field but not on the direction of lightpropagation. As used in the telecommunication industry, the Faradayrotator is essential to many devices that utilize its properties incombination with reciprocal polarization elements.

In the pass (forward) direction, light incident on the polarizer willpass through the polarizer without obstruction if its plane ofpolarization coincides with that of the polarizer. When this lightpasses through the Faraday rotator its plane of polarization is rotatedby 45° due to the magneto-optic effect. The direction of rotation, thatis, clockwise or counter clockwise, is dependent on the particularFaraday rotator configuration and is predetermined. The light thenpasses through the analyzer without loss, since the axis of polarizationof the analyzer is oriented at the same 45°.

In the blocking (reverse) direction, reflected light of arbitrarypolarization is incident on the analyzer which transmits some of thislight and polarizes it to match its axis of polarization. When thispolarized reflected light passes through the Faraday rotator its planeof polarization is again rotated by 45°, clockwise or counterclockwiserelative to the direction of light travel, as is predetermined. As aresult, the plane of polarization of the reflected light incident on thepolarizer is perpendicular to its axis of polarization, and, thus thereflected light is blocked by the polarizer. In this manner, the opticalisolator is used to transmit light from a source in the pass (forward)direction and essentially extinguish any reflected light in the blocking(reverse) direction. This extinguishing effect is commonly known as"isolation".

The magnitude of the rotation of the plane of polarization of lightpassing through the Faraday rotator depends on several factors, such as,the strength of the magnetic field, the nature of the material thatconstitutes the rotator, the frequency of the light, the temperature,and other parameters. Since the components in many optical applicationsutilizing the Faraday effect may be exposed to temperature variations,the rotational temperature dependency of the Faraday rotator limits theuse of Faraday rotators in devices which do not provide some form oftemperature compensation to prevent or minimize degradation inperformance.

The rotational temperature dependency of a Faraday rotator can beexpressed in terms of a temperature coefficient of rotation, C_(ROT),defined as: ##EQU1## where, θ is the rotation of the plane of polarizedlight passing through the Faraday rotator, and T is the temperature. Atypical Faraday rotator may have a temperature coefficient with amagnitude of as much as about 0.10°/° C. which can cause a variation ofFaraday rotation of about 12° over a temperature range of about -40° C.to 85° C. Of course, such undesirable rotation of the light can havesignificant detrimental effects on the performance of an optical deviceboth in terms of forward transmissivity and degree of reverse isolation.But, since isolation (attenuation in the blocking direction) is measuredvery close to zero, small changes can have orders of magnitude effectson the degree of isolation in terms of the blocking directiontransmission of reflected light.

One proposed solution to this problem is to provide temperaturecompensation via a cooling/heating source which maintains thetemperature of the Faraday rotator, and possibly the temperature of theentire device, including for example, the laser source, at the requiredvalue. This would require that the temperature of the Faraday rotator bemonitored and the output from the cooling/heating source be adjustedaccordingly. Thus, the components required in such a temperaturecompensation system would include a cooling/heating source, temperaturemeasurement device, a feedback system, and a power supply among others.Disadvantageously, such a temperature compensation scheme not only addsto the complexity and cost of the device, but, also to the size of theoptical device which can limit the use of the device in manyapplications.

In some cases, a cascaded isolator, such as a double stage isolator, isutilized to compensate for the effects of temperature variance onoptical isolators. Typically, a double stage isolator utilizes apolarizer, a Faraday rotator, an analyzer/polarizer, a second Faradayrotator, and a second analyzer arranged in this sequence. Thiseffectively provides two stages of optical isolators in series.Typically, to compensate for temperature variations, one stage is"de-tuned" to an offset temperature above the ambient temperature whilethe other stage is "de-tuned" to an offset temperature correspondinglybelow the ambient temperature so as to provide a more broad-bandresponse between the two temperature extremes. However, such detuningresults in overall degraded isolation performance over the temperaturerange and at the nominal design temperature. Another proposed solutionis to cascade multiple stages of isolators. But, the use of cascadedisolators in an optical device, undesirably, not only adds to thecomplexity, cost and size of the device, but, also increases the numberof components needed, and increases the optical path of the light whilereducing overall transmissivity through the cascaded isolators.

Optical components, such as the polarizer, the Faraday rotator, and theanalyzer of a typical optical compensator, are commonly fixed in anassembly or attached to a common substrate. The primary approach, in theindustry to date, to fixturing optical components involves theimplementation of screw-machined barrels or small blocks withcounter-bored features machined in. The optical components are placed inthese machined cavities which typically tend to be small in size (forexample, less than 2 mm in diameter). Not only is the machining processof generally tiny metal fixtures a costly and time consuming operation,but, also the discrete approach of fixing the optical components isgenerally not suited for mass automation. Undesirably, such a method offixturing optical components is labor intensive and leads to highermanufacturing costs and lower manufacturing efficiency.

Thus, there is a need for providing a Faraday rotator temperaturecompensator that is simple, low cost and dimensionally small and thereis a need to provide an efficient and low cost method that is welladapted for the automated manufacturing of such optical assemblies andother small components.

SUMMARY OF THE INVENTION

An optical isolator constructed in accordance with one preferredembodiment of the present invention overcomes some or all of theafore-mentioned disadvantages. The optical isolator utilizes theexpansion/contraction properties of a bimetallic element to compensatefor the effect temperature on Faraday rotation. The method also employs,in one embodiment, a lamination manufacturing procedure to create anarray of optics-receiving micro-fixtures for receiving or formingoptical assemblies.

The present invention provides thermal compensation in devices utilizingmagneto-optical materials, such as Faraday rotators, by utilizing theopening and closing arcing motion of coiled bimetallic metal strips dueto their expansion/contraction when exposed to temperature variations. Apolarization element is attached to a bimetallic element which allowscorrection for temperature induced Faraday rotation. The bimetallicelement is configured to optimally match the degree oftemperature-induced drift in the Faraday rotation with the rotation ofthe polarization element.

In one preferred embodiment of the present invention a temperaturecompensated optical isolator is provided. Preferably, the opticalisolator includes a pair of polarization elements, a Faraday rotator anda bimetallic element. One of the polarization elements is an opticalpolarizer while the other polarization element is an optical analyzer.The Faraday rotator is positioned between the polarizer and analyzer.The bimetallic element is attached between the analyzer and a base whichhouses the polarizer, Faraday rotator, analyzer and bimetallic element.

Advantageously, the bimetallic element of the present invention isconfigured to optimally conform with the temperature induced changes inFaraday rotation. By rotating the axis of polarization of the analyzerthe bimetallic element ensures that any back-reflected light incident onthe polarizer has a plane of polarization substantially perpendicular tothe polarizer's axis of polarization. Thus, all or most of theback-reflected light incident on the polarizer will be effectivelyextinguished, thereby essentially eliminating any temperature-induceddegradation in the effective isolation of the optical system.

Advantageously, the bimetallic element of the present invention can betailored to meet the particular characteristics of the magneto-opticalmaterial and is hence adaptable to a wide variety of situations andapplications. This is accomplished by appropriate material selection,configuration and dimensioning of the bimetallic element. In onepreferred embodiment of the optical isolator of the present invention,the bimetallic element has a generally curved portion which generallycircumscribes the analyzer. Of course, other shapes and configurationsmay also be employed with efficacy, as required or desired, giving dueconsideration to the goal of optimally enhancing the isolationperformance of the optical isolator over a given range of temperatures.Also, the bimetallic element can be used to house a variety ofpolarization elements and to provide temperature compensation in otheroptical devices. For example, the bimetallic element may be used inconjunction with a half-wave plate or it may be used in combination withan external modulator, to achieve some or all of the benefits andadvantages disclosed herein.

Advantageously, the optical isolator of the present invention provides asignificant improvement over conventional single stage isolators byessentially eliminating the effects of temperature on isolation. It iseffective in maintaining a consistent optical isolation over an extendedtemperature range thereby, allowing the optical device to functionwithout costly active temperature control. The isolator of the presentinvention also provides several advantages over conventionaldouble-stage or cascaded isolators. Desirably, it is lower in cost,simpler in design, dimensionally smaller, is easier to manufacture, andprovides a shorter optical path. This simplicity and compactness renderthe isolator of the present invention a viable choice for providingtemperature compensation in a wide variety of optical devices. The sizeof the isolator allows it to readily fit into standard optical packages.Additionally, the simple construction of the isolator make it apractically effortless retrofit into conventional opto-electronicpackages. Also, advantageously, the isolator of the present invention isenvironmentally stable and is well suited for the present and future inthe field of telecommunications.

The present invention also prescribes, in accordance with oneembodiment, a preferred method of manufacturing sub-assemblies ofoptical elements, such as in one embodiment the temperature compensatedoptical isolator. Preferably, the method utilizes lamination or layeringof sheets with arrays of micro-frames to form an array ofoptic-receiving micro-fixtures. The micro-frames are, preferably,photo-chemically etched or stamped into the sheets and are supported bytab members. Appropriate optical elements are inserted into themicro-fixtures as dictated by the particular application. The laminateunits or pallets which accommodate the array of optical elements arestacked, aligned and attached to form an array of optical assemblies ina laminate stack. A base member may be attached to the opticalassemblies to facilitate their mounting. The optical assemblies areremoved from the laminate stack by conventional trimming methods.

Advantageously, such a method is well suited for the automatedmanufacturing of optical assemblies and results in high speed, highvolume production, thereby desirably maintaining low manufacturingcosts. For example, the method can be used in combination withconventional pick-and-place type of robotics. The present methodprovides an improvement over conventional manufacturing of opticalassemblies which typically utilizes a laborious, time-consuming andcostly machining process.

Advantageously, the method can be customized to form or assemble a widevariety of optical components and other small components, and isadaptable to a wide range of applications. For example, the method maybe used to mount lenses, crystals, gratings, filters, fibers and varioussub-assemblies, among others.

For purposes of summarizing the invention and the advantages achievedover the prior art, certain objects and advantages of the invention havebeen described herein above. Of course, it is to be understood that notnecessarily all such objects or advantages may be achieved in accordancewith any particular embodiment of the invention. Thus, for example,those skilled in the art will recognize that the invention may beembodied or carried out in a manner that achieves or optimizes oneadvantage or group of advantages as taught herein without necessarilyachieving other objects or advantages as may be taught or suggestedherein.

All of these embodiments are intended to be within the scope of theinvention herein disclosed. These and other embodiments of the presentinvention will become readily apparent to those skilled in the art fromthe following detailed description of the preferred embodiments havingreference to the attached figures, the invention not being limited toany particular preferred embodiment(s) disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic drawing illustrating the pass-direction operationof a conventional optical isolator, as known in the art;

FIG. 1B is a schematic drawing illustrating the blocking-directionoperation of the isolator of FIG. 1A;

FIG. 2A is a graphical comparison of the theoretical isolationperformances, as a function of temperature, between the isolator of FIG.1 and one preferred embodiment of the isolator of the present invention;

FIG. 2B is a graphical comparison of the theoretical isolationperformances, as a function of Faraday rotation, between the isolator ofFIG. 1 and one preferred embodiment of the isolator of the presentinvention;

FIG. 2C is a graphical illustration of the theoretical transmission lossthrough one preferred embodiment of the isolator of the presentinvention;

FIG. 2D is a graphical illustration of the experimental isolationperformances, as a function of temperature, between the isolator of FIG.1 and one preferred embodiment of the isolator of the present invention;

FIG. 3 is a schematic illustration of a temperature compensated opticalisolator constructed in accordance with one preferred embodiment of thepresent invention;

FIG. 4A is a front elevational view illustrating one preferredbimetallic element of the isolator of FIG. 3;

FIG. 4B is a front elevational view illustrating another preferredbimetallic element of the isolator of FIG. 3;

FIG. 5 is an exploded perspective view of the isolator of FIG. 3;

FIG. 6 is a front elevational view illustrating the rotation of thebimetallic element of FIG. 4A;

FIG. 7A is a schematic drawing illustrating the pass-direction operationof the isolator of FIG. 3, at an optimized temperature;

FIG. 7B is a schematic drawing illustrating the blocking-directionoperation of the isolator of FIG. 3, at an optimized temperature;

FIG. 8A is a schematic drawing illustrating the pass-direction operationof the isolator of FIG. 3, at a temperature higher than the optimizedtemperature;

FIG. 8B is a schematic drawing illustrating the blocking-directionoperation of the isolator of FIG. 3, at a temperature higher than theoptimized temperature;

FIG. 9A is a schematic drawing illustrating the pass-direction operationof the isolator of FIG. 3, at a temperature lower than the optimizedtemperature;

FIG. 9B is a schematic drawing illustrating the blocking-directionoperation of the isolator of FIG. 3, at a temperature lower than theoptimized temperature;

FIG. 10 is a schematic illustration of the bimetallic element of FIG. 3in combination with a half-wave plate and a Faraday rotator;

FIG. 11 is a schematic illustration of a micro-framed sheet inaccordance with one preferred embodiment of the method of the presentinvention;

FIG. 12A is a front elevational view of one preferred micro-frame of thesheet of FIG. 11;

FIG. 12B is a front elevational view of another preferred micro-frame ofthe sheet of FIG. 11;

FIG. 13 is a front elevational view showing a preferred pair of tabmembers of the sheet of FIG. 11;

FIG. 14A is an exploded perspective view illustrating a preferred stepof laminating the sheets of FIG. 11;

FIG. 14B is a perspective view of the laminate unit (pallet) formed bythe step of FIG. 14A;

FIG. 15A is an exploded perspective view of a preferred micro-fixtureformed by the step of FIG. 14A;

FIG. 15B is a perspective view of an assembled micro-fixture formed fromthe components of FIG. 15A;

FIG. 15C is a perspective view of the micro-fixture of FIG. 15A incombination with an optical element;

FIG. 15D is a sectional view, taken along line 15D--15D of FIG. 15C;

FIG. 16A is an exploded perspective view illustrating a preferred stepof laminating the laminate units (pallets) of FIG. 14B;

FIG. 16B is a perspective view of the laminate stack formed by the stepof FIG. 16A;

FIG. 16C is a sectional view, taken along line 16C--16C of FIG. 16B;

FIG. 17 is a perspective view of a base employed in one preferredembodiment of the method of the present invention;

FIG. 18 is a front elevational view of another preferred fixture formedby the step of FIG. 14A;

FIG. 19A is a front elevational view of yet another preferred fixtureformed by the step of FIG. 14A; and

FIG. 19B is a sectional view, taken along 19B--19B of FIG. 19A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1A and 1B schematically illustrate the operating principle of aconventional optical isolator 10', as is well known in the art. Theisolator 10' includes a polarizer (polarizing element) 12, a Faradayrotator 14 and an analyzer (polarizing element) 16. In the pass(forward) direction, as illustrated in FIG. 1A, the optical isolator 10'permits the incident light 18 to be transmitted through it resulting ina beam of transmitted light 20. In the blocking (reverse) direction, asillustrated in FIG. 1B, the isolator 10' ideally blocks all or most ofthe back-reflected light 22, while operating at the optimizedtemperature, so that the reflection-induced light 24 passing through theisolator 10' is essentially none or negligible.

As illustrated in FIG. 1A, in the pass (forward) direction, incidentlight 18 passes through the polarizer 12 without obstruction since itsplane of polarization 26 coincides with the axis of polarization of thepolarizer 12. The Faraday rotator 14 rotates the plane of polarization26 of the incident light 18 by an angle β=45°, in a clockwise direction,so that the light that is transmitted through the Faraday rotator 14 hasa plane of polarization 28. Since the axis of polarization of theanalyzer 16 is oriented the same as the plane of polarization 28, all ormost of the light incident on the analyzer 16 passes through it astransmitted light 20.

In the blocking (reverse) direction, as illustrated in FIG. 1B, theback-reflected light 22 of arbitrary polarization 30 is incident on theanalyzer 16 which transmits some of this light and polarizes it to aplane of polarization 28 that matches the analyzer axis of polarization.Of course, the plane of polarization 28 of the reflected light 22 willbe oriented at 45° (clockwise) with respect to the plane of polarization26 of the incident light 18 (FIG. 1A). The Faraday rotator 14 rotatesthe plane of polarization 28 of the reflected light 22 by an angleβ=45°, again in a clockwise direction. Therefore, the plane ofpolarization 32 of the back-reflected light 22 that is transmittedthrough the Faraday rotator 14 is oriented at 90° with respect to theplane of polarization 26 of the incident light 18 and is, hence,perpendicular to the axis of polarization of the polarizer 12. Thisideally blocks all or most of the back-reflected light 22 so that thereflection-induced light 24 passing through the polarizer 12 isessentially none or negligible. In this manner, the optical isolator 10'is used to transmit light 18 from a source, such as a laser (not shown)in the pass (forward) direction and essentially extinguish any reflectedlight 22 in the blocking (reverse) direction.

This extinguishing effect is commonly known as "isolation". Referring tothe blocking (reverse) direction shown in FIG. 1B, the isolation isdefined as the amount of unpolarized light incident on the analyzer 16that passes through the polarizer 12. The effective transmission, τ,through a polarization element can be defined as:

    τ=k.sub.1 cos.sup.2 (φ)+k.sub.2 sin.sup.2 (φ)  (1)

where, k₁ is the maximum transmission of polarized light through thepolarizing element, k₂ is the minimum transmission of polarized lightthrough the polarizing element, and φ is the angular offset between theplane of polarization of the polarized light and the axis ofpolarization of the polarization element. When φ=0°, that is when theplane of polarization of the polarized light coincides with the axis ofpolarization of the polarization element, the transmission through thepolarization element will be maximum and given by τ=k₁. When φ=90°, thatis when the plane of polarization of the polarized light isperpendicular to the axis of polarization of the polarization element,the transmission through the polarization element will be minimum andgiven by τ=k₂.

Of course, k₁ and k₂ are generally dependent on the characteristics ofthe particular polarizing element, as is well known in the art. Forexample, typically, k₁ is about 0.98 and k₂ is about 0.000098 for highcontrast POLARCOR™ polarizers from Corning, Inc. of Coming, N.Y. Toillustrate the effect of temperature on the isolation performance of theoptical isolator 10' (FIGS. 1A and 1B) it is assumed that the polarizer12 and the analyzer 16 are POLARCOR™ polarizers with k₁ =0.98 and k₂=0.000098, and that there is no transmission loss through the Faradayrotator 14.

Referring to FIG. 1B, and using equation (1), the effectivetransmission, τ₁₆, of the unpolarized reflected light 22 with arbitrarypolarization 30 through the analyzer 16 is given by:

    τ.sub.16 =k.sub.1 cos.sup.2 (φ)+k.sub.2 sin.sup.2 (φ)=1/2(k.sub.1 +k.sub.2)                             (2)

since, the average value of cos² (φ) and sin² (φ), over 0°≦φ≦360°, is1/2. As mentioned above, the light that is transmitted through theanalyzer 16 is linearly polarized by the analyzer 16.

Referring to FIG. 1B, after the plane of polarization of the polarizedreflected light has been rotated by the Faraday rotator, the rotatedpolarized light is incident on the polarizer 12. Using equation (1), theeffective transmission, τ₁₂, of the polarized reflected light with planeof polarization 32 through the polarizer 12 is given by:

    τ.sub.12 =k.sub.1 cos.sup.2 (φ)+k.sub.2 sin.sup.2 (φ)=k.sub.1 cos.sup.2 (90°)+k.sub.2 sin.sup.2 (90°)=k.sub.2(3)

since, the plane of polarization 32 is perpendicular to the axis ofpolarization of the polarizer 12.

The isolation transmission, τ_(ISO), through the optical isolator 10'(see FIG. 1B), neglecting any loss through the Faraday rotator 14, is:

    τ.sub.ISO =τ.sub.16 τ.sub.12                   (4)

The degree or amount of isolation, I, of the optical isolator 10' canalso be represented in terms of deciBels (dB) by:

    I=-10 log(τ.sub.ISO)                                   (5)

It is desirable to have a high isolation, typically at least about 40dB, so that reflection induced noise due to the optical isolator isminimized in the optical device, such as a laser.

For POLARCOR™ polarizers, with k₁ =0.98 and k₂ =0.000098, τ₁₆ is0.490049 using equation (2), τ₁₂ is 0.000098 using equation (3), so thatthe isolation transmission, τ_(ISO), is 4.8024802×10⁻⁵ from equation(4). Thus, using equation (5) the isolation, I, is 43.185 dB, therebyeffectively extinguishing transmission of most back-reflected lightthrough the optical isolator. Of course, this analysis neglects theeffect of any Faraday rotator temperature variations and assumes opticalisolator operation at an optimum temperature, typically 25° C.

As mentioned above, each Faraday rotator has a temperature coefficientof rotation. Thus, changes in temperature influence the magnitude ofFaraday rotation and can undesirably lead to degradation in isolationperformance. For example, if the temperature of the Faraday rotator(FIGS. 1A and 1B) changes from the optimum operating temperature, theplane of polarization 32 (FIG. 1B) of the back-reflected light incidenton the polarizer 12 will be offset somewhat from the desired 90°. Thiswill result in an increase in the effective transmission τ₁₂ ofreflected light back through the polarizer 12, and hence, an increase inthe noise or isolation transmission τ_(ISO) and a correspondingundesirable decrease in the isolation, I, of the optical isolator 10'.

This effect of temperature on the isolation of the optical isolator 10'(FIGS. 1A and 1B), calculated by using equations (1) to (5), isillustrated in FIGS. 2A and 2B by respective curves 34' and 36'. Forillustration purposes, it is assumed that the Faraday rotatortemperature coefficient of rotation, C_(ROT), is -0.1°/° C.

The graph of FIG. 2A shows the effect of temperature on the isolationwith the x-axis 38 representing the temperature and the y-axis 40representing the amount of isolation, I. As can be seen by curve 34' inFIG. 2A, the isolation of the optical isolator 10' (FIGS. 1A and 1B) isa maximum at the optimum operating temperature of 25° C. But, as thetemperature varies from 25° C., the isolation, undesirably, dropssharply. Referring to curve 34', a temperature variation of about 70° C.causes a drop of more than 20 dB in the amount of isolation. Thisrepresents an almost 150-fold increase in the amount of back-reflectednoise, τ_(ISO), admitted into the optical circuit.

The graph of FIG. 2B shows a similar effect on the isolation in terms ofthe rotation, θ, through the Faraday rotator (FIGS. 1A and 1B) with thex-axis 42 representing the Faraday rotation and the y-axis 44representing the amount of isolation. As can be seen by curve 36' inFIG. 2B, the isolation of the optical isolator 10' (FIGS. 1A and 1B) isa maximum at the optimum Faraday rotation of 45°. But, as the rotationvaries from 45°, due to the temperature variation, the isolation,undesirably, drops sharply. Referring to curve 36', a Faraday rotationvariation of about 7° causes a drop of more than 20 dB in the amount ofisolation. This represents an almost 150-fold increase in the amount ofback-reflected noise τ_(ISO) admitted into the optical circuit.

The temperature variation will also affect the transmission of lightthrough the isolator 10' in the pass (forward) direction shown in FIG.1A. The transmission in the pass (forward) direction will be reducedsince the light incident on the analyzer 16 (FIG. 1A) will have a planeof polarization that is offset from the axis of polarization of theanalyzer, due to the temperature induced variation in Faraday rotation.This transmission loss in deciBels can be determined by using equation(5), and is typically small. Thus, temperature variations, are criticalin undesirably degrading the isolation performance of the prior artoptical isolator 10', while having a less significant effect on theforward transmission.

Temperature Compensated Isolator

The present invention provides thermal compensation for magneto-opticalmaterials, such as a Faraday rotator, by utilizing the arcing or curlingmotion of bimetallic metal strips due to their expansion/contractionwhen exposed to temperature variations. A polarization element isattached to a bimetallic element which allows correction for temperatureinduced Faraday rotation. The bimetallic element is configured tooptimally match the Faraday rotation with the polarization elementrotation.

FIG. 3 schematically illustrates one preferred embodiment of atemperature compensated optical isolator 10 including a polarizer orpolarization element 12, a magneto-optical element or Faraday rotator14, and an analyzer or a polarization element 16 which is attached to abimetallic element 46. FIG. 4A is a detailed front view depicting howthe analyzer 16 is attached to the bimetallic element 46. Those skilledin the art will readily comprehend that as the ambient temperatureincreases or decreases the bimetallic element will expand or contractcausing the analyzer 16 to be rotated in a clockwise or counterclockwisedirection. The arcing or curling motion of the bimetallic element 46causes the analyzer 16, and hence its axis of polarization 72, to berotated about an axis 54 (shown in FIGS. 3, to 6) that is parallel tothe direction of propagation of the light incident on the isolator 10and generally passes through the center of the polarizer 12, Faradayrotator 14 and analyzer 16. This rotation is relative to the axis ofpolarization 74 of the polarizer 12 and is illustrated in FIG. 4A by theposition of the analyzers labeled 16a and 16b. Under optimum temperatureconditions, typically 25° C., the axes of polarization 72 and 74 areangularly offset by about 45° and no temperature compensation isrequired, as discussed above.

Referring to FIG. 4A, preferably, the bimetallic element 46 includes apair of generally straight portions 48, 52 which are joined by agenerally curved portion 50. The analyzer 16 is attached to thebimetallic element straight portion 48 such that the bimetallic element46 generally circumscribes it while allowing the analyzer 16 to freelyrotate as the bimetallic element expands or contracts. Preferably, theanalyzer 16 is glued to the bimetallic element 46 utilizingpad-printable B-stageable epoxy #118-03 available from CreativeMaterials Inc. of Tyngsboro, Mass. Alternatively, the analyzer 16 may beattached to the bimetallic element 46 using EP353-ND epoxy availablefrom Epoxy Technology of Billerica, Mass, Optionally, other types ofglues or other attachment means, such as pins, locks, clamps, solderingor brazing among others, may be used with efficacy to affix the analyzer16 to the bimetallic element 46 giving due consideration to the desiredgoal of providing a reliable, clean, inert and generally temperatureindependent attachment. In one preferred embodiment of the presentinvention the edge of the analyzer 16 is metallized with layers ofchromium, nickel and gold using vacuum deposition. This permits theanalyzer 16 to be attached to the bimetallic element 46, as illustrated,for example, in FIG. 5, by soldering.

Preferably, and referring to FIGS. 4A and 5, the bimetallic elementstraight portion 52 is attached to a base or support 56. In onepreferred embodiment of the present invention, the base 56 is generallyyoke-shaped with a generally U-shaped cavity 64 in communication with agenerally circular cavity 62 for housing the polarizer 12 and Faradayrotator 14. Of course, the base 56 can be alternately configured asneeded or desired, giving due consideration to the goals of providing asupport for the isolator 10 and for permitting the isolator 10 to bereadily mounted in optical devices. The bimetallic element straightportion 52 resides inside the cavity 64 and is attached to an innersurface 60 of a wall 58 of the base 56. The flat surface feature of theinner surface 60 facilitates this attachment. Preferably, the bimetallicelement straight portion 52 is laser welded to the base 56, though otherattachment means, such as resistance welding, soldering or gluing, forexample, using epoxy, may be utilized with efficacy. In an alternativeembodiment of the present invention, as illustrated in FIG. 4B, a curvedportion 50' of a bimetallic element 46' is attached to a groove 112 in abase wall 58' while a flat portion is affixed to the analyzer 16.

Referring particularly to FIG. 5, preferably, the polarizer 12 and theFaraday rotator 14 are mounted in fixtures 66 and 68, respectively,using the B-stageable epoxy. Alternatively, other types of glues, suchas the EP353-ND epoxy, or other attachment means, such as pins, locksand clamps among others, may be used with efficacy to affix thepolarizer 12 and Faraday rotator 14 to the respective fixtures 66, 68giving due consideration to the desired goal of providing a reliable,clean, inert and generally temperature independent attachment. Thepolarizer fixture 66 and the Faraday rotator fixture 68 are configuredto fit in the generally circular base cavity 62, and define an aperture70 (see FIG. 4A) through which light can pass. Advantageously, theoptical isolator 10 is configured so that the entire field of view ofthe aperture 70 lies within the analyzer 16, even when the analyzer 16rotates to provide temperature compensation. Preferably, the polarizerfixture 66 and Faraday rotator fixture 68 are laser welded to the base56, though other attachment means, such as resistance welding, solderingor gluing, for example, using epoxy, may be utilized with efficacy. Thepolarizer fixture 66 and Faraday rotator fixture 68 may also be affixedto one another by one or more of the attachment means discussed above.

Referring to FIGS. 3 to 5, preferably, the polarizer 12 and the analyzer16 are high contrast POLARCOR™ polarizing elements with a V2anti-reflecting coating, and are available from Corning Inc. of Corning,N.Y. The particular wavelength or range of wavelengths of theapplication dictates the selection of the POLARCOR™ element. Preferably,the Faraday rotator 14 is a Latching Gamet Film #L22, which in use doesnot require an external magnetic field. Such devices are available fromLucent Technologies of Murray Hill, N.J. Again, the wavelength selectionis dictated by the requirements of the particular application. TheFaraday rotator may be fabricated from a variety of othermagneto-optical materials to achieve the benefits and advantagesdisclosed herein. A non-latching garnet may also be used as the Faradayrotator 14 and the entire isolator assembly housed in a magnet or themagnet may be shaped into a base, such as the base 56. The base 56 ispreferably fabricated from stainless steel, though other alloys, metals,plastics and ceramics may be utilized with efficacy, as required ordesired. The base 56 may be manufactured by machining, molding, forgingor casting. In one preferred form of the present invention, the base 56may be molded from a high temperature engineering plastic, such asnylon, teflon, polyetheretherketone (PEEK) or phenolic.

Referring in particular to FIG. 6, preferably, the bimetallic element 46is fabricated from ASTM #TM2, available from Atlantic Alloys of Bristol,R.I. In one preferred form of the invention, the ASTM #TM2 has an innerhigh expansion side (HES) 47 with a 55% layer ratio and a chemicalcomposition of 72 Mn, 18 Cu, 10 Ni, and an outer low expansion side(LES) 49 with a 45% layer ratio and a chemical composition of 36 Ni, 64Fe (Invar). The ASTM #TM2 is nickel plated for corrosion resistance. Inone preferred form of the present invention, this nickel plating alsopermits the analyzer 16 to be soldered to the bimetallic element 46, asindicated above. Those skilled in the art will readily comprehend thatthe selection of the bimetallic element 46 is largely dictated by thetemperature range of interest, and by the particular configuration andcomponents of the optical isolator 10 and their behavioral variancecharacteristics over that temperature range. Thus, other types ofbimetallic elements with alternate layer ratios and chemicalcompositions may be used with efficacy, as required or desired, givingdue consideration to the goal of providing optimal temperaturecompensation and maximum isolation.

Referring to FIG. 6, the angular rotation, A, of the bimetallic element46 is dependent not only on the material specification of the bimetallicelement 46, but also on its active length, L, and thickness, t. Theangular rotation, A, can be approximated by: ##EQU2## where, F is theflexivity of the materials comprising the bimetallic element 46 and ΔTis the temperature change or variation from the optimum operationaltemperature, which is typically 25° C. Desirably, the angular rotation,A, is linearly dependent on the temperature change, ΔT, as can be seenfrom equation (6). Additionally, the temperature coefficient ofrotation, C_(ROT), of most Faraday rotator's remains approximatelyconstant as a function of temperature, that is, the change in Faradayrotation is approximately linearly dependent on the temperature change.Advantageously, this permits, by proper selection of the material anddimensions of the bimetallic element 46, the temperature induced changein Faraday rotation to be matched by a generally corresponding angularrotation of the bimetallic element 46. Thus, the analyzer 16 which isattached to the bimetallic element 46, as shown in FIG. 4A, undergoes acorresponding rotation which substantially compensates for undesirablethermally induced back-reflection, as will be discussed at greaterlength later herein.

In one preferred form of the invention, the optical isolator 10 (shown,for example, in FIG. 5) is dimensioned and configured to optimallycompensate for temperature variations in the range of -40° C. (-40° F.)to 85° C. (185° F.). The bimetallic element 46 is ASTM #TM2 as discussedabove and its average flexivity is approximately 186×10⁻⁷ mm/mm/° C.(203×10⁻⁷ in/in/° F.) in the above temperature range of interest. Thoseskilled in the art will realize that the flexivity value will also havesome temperature dependency. But, advantageously, the deviation inflexivity of ASTM #TM2 from the average value is small. In the abovetemperature range the flexivity value changes in the range from about+1% to about -6%.

Referring to FIGS. 3 to 6, preferably, the optical isolator 10 isconfigured so that the respective axes of polarizations 74, 72 of thepolarizer 12 and analyzer 16 are angularly offset by 45° at a desirednominal design temperature, such as 25° C. In this optimumconfiguration, at a temperature of 25° C., the analyzer 16 and thebimetallic element 46 are positioned as illustrated in FIGS. 4A and 6.At 25° C., the curved portion 50 of the bimetallic element 46 defines anincluded angle of about 218° and has an internal diameter of about 1.905mm (0.075 inches). The thickness of the bimetallic element is about0.1016 mm (0.004 inches) and its width is about 0.7874 mm (0.031inches). With these dimensions, the active length, L, of the bimetallicelement 46 is readily calculated to be about 3.81 mm (0.150 inches).

Referring to FIG. 6, as the temperature increases above the optimumtemperature of 25° C., the bimetallic element 46 curls outwards orexpands as illustrated by the position of the bimetallic element 46a.Using equation (6), at the upper temperature extreme of 85° C. (185°F.), the bimetallic element 46 will undergo an angular rotation, A, ofapproximately +6° about the axis 54. Referring to FIG. 4A, the analyzer16 will experience a corresponding rotation of approximately +6° aboutthe axis 54 and relative to the polarizer axis of polarization 74, asillustrated by the position of the analyzer 16a, so that the axes ofpolarization 72, 74 are angularly offset by about 51°.

Referring to FIG. 6, as the temperature decreases below the optimumtemperature of 25° C., the bimetallic element 46 curls inwards orcontracts as illustrated by the position of the bimetallic element 46b.Using equation (6), at the lower temperature extreme of -85° C. (-185°F.), the bimetallic element 46 will undergo an angular rotation, A, ofapproximately -6° about the axis 54. Referring to FIG. 4A, the analyzer16 will experience a corresponding rotation of approximately -6° aboutthe axis 54 and relative to the polarizer axis of polarization 74, asillustrated by the position of the analyzer 16b, so that the axes ofpolarization 72, 74 are angularly offset by about 39°.

Referring to FIG. 4A, the analyzer 16 will also be axially displaced asit undergoes temperature induced rotation. It is desirable to optimallyreduce this axial displacement. Preferably and advantageously, thecomponents of the optical isolator 10 (FIGS. 3 to 6) are configured sothat the entire field of view of the aperture 70 lies within theanalyzer 16, even when the analyzer 16 is rotationally and axiallydisplaced to the extreme positions 16a and 16b, shown in FIG. 4A. Toaccomplish this, in one preferred form, the analyzer 16 is approximatelycentered on the axis 54 and is generally square shaped with an about1.27 mm (0.050 inches) side, and the polarizer 12, polarizer fixture 66,Faraday rotator 14, Faraday rotator fixture 68 are configured anddimensioned to provide an aperture 70 that is approximately centered onthe axis 54 and is generally circular with a diameter of about 1.016 mm(0.040 inches). In one preferred form of the present invention, thePOLARCOR™ polarizer 12 and the POLARCOR™ analyzer 16 are about 200 μmthick and the Latching Garnet Faraday rotator 14 is about 300 to 500 μmthick, though those skilled in the art will be aware that the selectionof these dimensions is generally wavelength dependent. Referring to FIG.4A, the base 56 has a length L of about 2.159 mm (0.085 inches), a widthW of about 2.489 mm (0.098 inches), and a depth that is selected toaccommodate the components of the isolator 10. It will be apparent tothose skilled in the art that the scope of the present inventionincludes alternate configurations and dimensions, as needed or desired,to achieve the benefits and advantages disclosed herein. These alternateconfigurations and dimensions will at least partly be governed by theapplication. In particular, the material selection and dimensioning ofthe bimetallic element 46 can be used to provide temperaturecompensation for a wide variety of Faraday rotators having varyingtemperature coefficients of rotation, C_(ROT). Also, the temperaturecompensation means of the present invention can be used to rotate othercomponents or combination of components of the isolator 10 to achievethe same effect. For example, the polarizer 12 may be rotated, both thepolarizer 12 and the Faraday rotator 14 may be rotated, both thepolarizer 12 and analyzer 16 may be rotated, or other combinationsthereof giving due consideration to the goal of optimally enhancing theisolation performance.

The operation of the temperature compensated optical isolator 10 is bestillustrated by reference to FIGS. 7A to 9B. FIGS. 7A and 7B show theoperation of the optical isolator 10 in the pass (forward) direction andin the blocking (reverse) direction, respectively, at an optimizedtemperature, typically 25° C. This is similar to the operation of theprior art isolator 10' shown in FIGS. 1A and 1B, at the optimizedtemperature. FIGS. 8A and 8B show the operation of the optical isolator10 in the pass (forward) direction and in the blocking (reverse)direction, respectively, at a temperature higher than the optimum designtemperature. Similarly, FIGS. 9A and 9B show the operation of theoptical isolator 10 in the pass (forward) direction and in the blocking(reverse) direction, respectively, at a temperature lower than theoptimum design temperature.

As illustrated in FIG. 7A, in the pass (forward) direction at theoptimized temperature, the polarized incident light 20 with the plane ofpolarization 26 is transmitted without obstruction through the polarizer12 with axis of polarization 74, since the plane of polarization 26matches the polarizer axis of polarization 74. As the light passesthrough the Faraday rotator 14, the plane of polarization 26 is rotatedby an angle β=45° to form the plane of polarization 28. Since the axisof polarization 72 of the analyzer 16 is angularly offset by β=45° fromthe polarizer axis of polarization 74, and thus oriented the same as theplane of polarization 28, all or most of the light incident on theanalyzer 16 passes through it as transmitted light 20 with a plane ofpolarization 29 which is the same as the plane of polarization 28.

In the blocking (reverse) direction at the optimized temperature, asillustrated in FIG. 7B, the back-reflected light 22 of arbitrarypolarization 30 is incident on the analyzer 16 which transmits some ofthis light 22 and polarizes it to a plane of polarization 29 (or 28)that matches its axis of polarization 72. The Faraday rotator 14 rotatesthe plane of polarization 29 (or 28) by an angle β=45° to the plane ofpolarization 32 which is desirably oriented at 2β=90° with respect tothe axis of polarization 74 of the polarizer 12. Thus, anyreflection-induced light 24 passing through the polarizer 12 isessentially none or negligible.

FIGS. 8A and 8B illustrate the operation of the optical isolator 10 at atemperature higher than the optimized temperature. Referring to FIG. 8A,in the pass (forward) direction, the polarized incident light 20 with aplane of polarization 26 is transmitted without obstruction through thepolarizer 12 with axis of polarization 74, since the plane ofpolarization 26 matches the polarizer axis of polarization 74. Asmentioned before, the temperature variation affects the rotation ofpolarized light through the Faraday rotator 14 due to an inherentFaraday rotation temperature coefficient of rotation, C_(ROT). As thelight passes through the Faraday rotator 14, the plane of polarization26 is rotated by an angle (β-Δβ) to form the plane of polarization 28',where β=45° and Δβ is the temperature induced perturbation to theFaraday rotation. In one preferred form of the invention, Δβ isapproximately 6° at a high temperature extreme of about 85° C. (185°F.), so that (β-Δβ) is about 51°. Due to the increase in temperature,relative to the optimized temperature, the analyzer 16 attached to thebimetallic element 46 is rotated so that its axis of polarization 72' isangularly offset from the polarizer axis of polarization 74 by about(β+Δβ) and from the plane of polarization 28' by about 2Δβ. As a result,there will be some loss in transmission through the analyzer 16, butthis loss is generally small and acceptable, as discussed later. Also,the plane of polarization 29' of the transmitted light 20' will beangularly offset by about (β+Δβ) from the polarizer axis of polarization74 and by about 2Δβ from the plane of polarization 28'. This is incontrast to the transmission through the analyzer 16 at the optimumdesign temperature, as illustrated in FIG. 7A, wherein the light istransmitted with essentially no loss in transmission and the planes ofpolarization 28 and 29 are essentially the same.

In the blocking (reverse) direction, as illustrated in FIG. 8B, theback-reflected light 22' of arbitrary polarization 30' is incident onthe analyzer 16 which transmits some of this light 22' and polarizes itto a plane of polarization 29' that matches its axis of polarization72'. The light that is incident on the Faraday rotator 14 has a plane ofpolarization 29' which is oriented at an angle of (β+Δβ) with respect tothe polarizer axis of polarization 74. The Faraday rotator 14 rotatesthe plane of polarization 29' by an angle (β-Δβ). Thus, advantageously,the light that passes through the Faraday rotator 14 has a plane ofpolarization 32' which is desirably oriented at 2β=90° with respect tothe axis of polarization 74 of the polarizer 12. As a result, anyreflection-induced light 24' passing through the polarizer 12 isessentially none or negligible. In this manner, the temperaturecompensated optical isolator 10 of the present invention, by controllingthe relative orientation of the analyzer 16 with respect to thepolarizer 12 essentially eliminates all or most of the effect oftemperature increases on the performance of the isolator 10.

Similarly, FIGS. 9A and 9B illustrate the operation of the opticalisolator 10 at a temperature lower than the optimized temperature.Referring to FIG. 9A, in the pass (forward) direction, the polarizedincident light 20 with a plane of polarization 26 is transmitted withoutobstruction through the polarizer 12 with axis of polarization 74, sincethe plane of polarization 26 matches the polarizer axis of polarization74. As the light passes through the Faraday rotator 14, the plane ofpolarization 26 is rotated by an angle (β+Δβ) to form the plane ofpolarization 28", where β=45° and Δβ is the temperature inducedperturbation to the Faraday rotation. In one preferred form of theinvention, Δβ is approximately 6° at a low temperature extreme of about-40° C. (-40° F.), so that (β+Δβ) is about 45°. Due to the decrease intemperature, relative to the optimized temperature, the analyzer 16attached to the bimetallic element 46 is rotated so that its axis ofpolarization 72" is angularly offset from the polarizer axis ofpolarization 74 by about (β-Δβ) and from the plane of polarization 28"by about 2Δβ. As a result, there will be some loss in transmissionthrough the analyzer 16, but this loss is generally small andacceptable, as discussed later. Also, the plane of polarization 29" ofthe transmitted light 20" will be angularly offset by about (β-Δβ) fromthe polarizer axis of polarization 74 and by about 2Δβ from the plane ofpolarization 28". This is in contrast to the transmission through theanalyzer 16 at the optimized temperature, as illustrated in FIG. 7A,wherein the light is transmitted with essentially no loss intransmission and the planes of polarization 28 and 29 are essentiallythe same.

In the blocking (reverse) direction, as illustrated in FIG. 9B, theback-reflected light 22" of arbitrary polarization 30" is incident onthe analyzer 16 which transmits some of this light 22" and polarizes itto a plane of polarization 29" that matches its axis of polarization72". The light that is incident on the Faraday rotator 14 has a plane ofpolarization 29" which is oriented at an angle of (β-Δβ) with respect tothe polarizer axis of polarization 74. The Faraday rotator 14 rotatesthe plane of polarization 29" by an angle (β+Δβ). Thus, advantageously,the light that passes through the Faraday rotator 14 has a plane ofpolarization 32" which is desirably oriented at 2β=90° with respect tothe axis of polarization 74 of the polarizer 12. As a result, anyreflection-induced light 24" passing through the polarizer 12 isessentially none or negligible. In this manner, the temperaturecompensated optical isolator 10 of the present invention, by controllingthe relative orientation of the analyzer 16 with respect to thepolarizer 12 essentially eliminates all or most of the effect oftemperature degradation in the performance of the isolator 10.

As can be deduced from FIGS. 7B, 8B and 9B, the degree of Faradayrotation at a given temperature in combination with the orientation ofthe analyzer 16 with respect to the polarizer 12 defines the amount ofisolation through the isolator 10. The following equation (7) representsthe condition of optimally maximizing the isolation of the opticalisolator 10:

    β.sub.Faraday (T)+β.sub.Analyzer (T)=90°  (7)

where, β_(Faraday), which is a function of temperature T, is the Faradayrotation at a temperature T and β_(Analyzer), which is also a functionof temperature T, is the angular offset between the analyzer axis ofpolarization 72 and the polarizer axis of polarization 74 at thetemperature T. Thus, as long as the sum of β_(Faraday) and β_(Analyzer)is about 90°, the back-reflected light that is incident on the polarizer12 will have a plane of polarization, for example, the plane ofpolarization 32' shown in FIG. 8B, that is perpendicular to thepolarizer axis of polarization 74. This, advantageously, ensures optimalmaximization of isolation of the optical isolator 10.

The optimum theoretical performance of the temperature compensatedoptical isolator 10 (as shown, for example, in FIG. 5) is illustrated inFIGS. 2A and 2B which also shows the theoretical isolation performance,as discussed before, of the prior art isolator 10' (shown in FIGS. 1Aand 1B). For illustration and comparison purposes it is assumed that theFaraday rotator temperature coefficient of rotation, C_(ROT), is about-0.1°/° C. and the polarizer 12 and the analyzer 16 are POLARCOR™polarizing elements, as described above. Equations (1) to (5) are usedto calculate the isolation, I, shown in the graphs of FIGS. 2A and 2B.

The graph of FIG. 2A shows the effect of temperature on the isolationwith the x-axis 38 representing the temperature and the y-axis 40representing the isolation. Curve 34 represents the theoreticalisolation of the temperature compensated optical isolator 10 (shown, forexample, in FIGS. 3 and 5) while the curve 34' represents the isolationof a conventional optical isolator 10' (shown in FIGS. 1A and 1B).Advantageously, as can be seen by curve 34, the isolation of the opticalisolator 10 of the present invention is essentially constant over atemperature range of -45° C. to 95° C. In contrast, anddisadvantageously, as can be seen by curve 34', the isolation of theconventional optical isolator 10' drops sharply as the temperaturevaries from the optimum design temperature of 25° C. The graph of FIG.2B shows a similar effect on the isolation in terms of the Faradayrotation, θ, with the x-axis 42 representing the Faraday rotation andthe y-axis 44 representing the isolation. Curves 36 and 36' representthe isolation of the isolators 10 and 10', respectively.

FIG. 2D shows an experimental comparison between the isolationperformance of a conventional optical isolator 10' (FIGS. 1A and 1B) andthe isolator (shown, for example, in FIGS. 3 and 5) of the presentinvention. Both isolators 10 and 10' include the same optical componentsbut the isolator 10 utilizes the temperature compensation scheme of thepresent invention. The graph of FIG. 2D shows the effect of temperatureon the isolation with the x-axis 114 representing the temperature (in °C.) and the y-axis 116 representing the isolation (in deciBels). Curve118 represents the isolation of the temperature compensated opticalisolator 10 as extrapolated from measured data while curve 118'represents the measured isolation of a conventional optical isolator10'. Advantageously, and as the experimental data of FIG. 2D shows, theisolation level of the isolator 10 is generally constant withtemperature and close to the accepted value of 40 dB. In contrast, anddisadvantageously, the isolation performance of the conventionalisolator 10' degrades sharply as the temperature drifts from the nominaldesign point of 25° C.

Referring in particular to the pass direction shown in FIGS. 8A and 9A,and as mentioned above, the light transmitted through the analyzer 16will experience some additional loss in transmission. Referring, forexample, to FIG. 8A, to compensate for the increase in temperature theanalyzer axis of polarization 72' is angularly offset from the plane ofpolarization 28' by an angle of 2Δβ, so that the isolation of theoptical isolator 10 is optimally maximized (as can be seen in the graphof FIG. 2A). For one preferred form of the invention as described above,at a high temperature extreme of 85° C. the change in Faraday rotationis about 6° so that 2Δβ is about 12°. Using equation (1), the analyzertransmission is about 0.94 compared to the optimum transmission at 25°C. of about 0.98. This calculated transmission loss, in deciBels, isshown in the graph of FIG. 2C, by curve 35, with the x-axis 39representing the input polarization angle and the y-axis 41 representingthe transmission loss in deciBels. The loss in transmission at thetemperature extremes of 85° C. (polarization input angle=12°) is onlyabout 0.26 dB. Similarly, the loss in transmission at a low temperatureextreme of -40° C. (polarization input angle=-12°) is only about 0.26dB. Even though this transmission loss is about twice that through theconventional optical isolator 10' (FIGS. 1A and 1B), it is a small lossin forward transmission and is within acceptable limits, given that theisolation performance of the temperature compensated optical isolator 10is optimally enhanced. Advantageously, the temperature compensatedoptical isolator 10 permits a small loss in forward transmission to gaina huge improvement in isolation performance.

Advantageously, the optical isolator 10 of the present inventionprovides consistent and effective optical isolation over an extendedtemperature, thereby desirably eliminating the need for costly activetemperature control. Moreover, the isolator 10 compared to conventionalcascaded isolators, is less expensive, has a shorter optical path, isdimensionally smaller, and easier to manufacture. The size of theisolator 10 allows it to readily fit into standard optical packages.Additionally, the simple construction of the isolator 10 make it apractically effortless retrofit into conventional opto-electronicpackages. Also, desirably, the present invention provides an isolator 10that is environmentally stable.

Advantageously, the bimetallic element 46 can be tailored tocorrespondingly conform to a particular Faraday rotator's temperaturecoefficient of rotation, by appropriate selection of materials,dimensions and configuration. This adaptability of the bimetallicelement 46 adds to the versatility of the isolator 10 and to any otherdevice utilizing the present temperature compensation scheme. Thetemperature compensation means of the present invention provide a simpleyet substantially accurate solution to optimally minimize thetemperature induced degradation in performance of optical devices whichutilize Faraday rotators.

Temperature Compensation Element

The bimetallic element 46 (see, for example, FIG. 6) can also be used tohouse other polarization elements. In one preferred embodiment of thepresent invention, schematically illustrated in FIG. 10, the bimetallicelement 46 is attached to a half-wave plate 76. The half-wave plate 10acts upon incident polarization by rotating its plane of polarization bytwice the angle between the plane of polarization and the slow-axis ofthe wave plate 76, as is known to those skilled in the art. Thisrotation, Ω, can be represented by:

    Ω=2φ                                             (8)

where, φ is the angular offset between the plane of polarization of theincident light and the slow-axis of the half wave plate 76.

Referring to FIG. 10, the half wave plate 76 is often used incombination with a Faraday rotator 14. In the forward direction, thiscombination can be used to rotate the plane of polarization of incidentlight by a total of 90°. In the reverse direction, the Faraday rotator12 and half wave plate 76 combination can be used to rotate thispolarization plane by a further 90°, in the opposite direction. In thismanner, the plane of polarization of the incident light is essentiallyrotated by about 0°. Such devices are often used in optical circulatorsto alter the optical path of light, by causing it to travel in oppositedirections, in association with other polarization optics. Of course,the bimetallic element 46 (see, for example, FIG. 6) providestemperature compensation in a manner similar to that discussed above, byadjusting the orientation of the half-wave plate 76 relative to theincident polarized light, thereby ensuring optimal performance.

It will be apparent to those skilled in the art, that the temperaturecompensation concept of the present invention can be applied to a rangeof other applications. For example, temperature compensation may beprovided for an external modulator in which a Faraday rotator is subjectto an alternating magnetic field to alternate the direction of Faradayrotation. Such modulators are used akin to a high-low type of switch.Temperature compensation for the modulator can be provided by housing apolarization element of the modulator in a bimetallic element. Byadjusting the axis of polarization of the polarization element theperformance of the modulator can be generally improved in-response totemperature variations.

Lamination-based Manufacturing Method

The present invention, in one embodiment, also prescribes a preferredmethod of manufacturing sub-assemblies of optical elements, such as thetemperature compensated optical isolator 10 (shown in, for example, FIG.5). Preferably, the method utilizes lamination or layering of sheetswith arrays of photo-chemically etched micro-frames which are used tohouse the optics. Advantageously, such a method is well suited forautomated manufacturing and results in high speed, high volumeproduction, thereby desirably maintaining low manufacturing costs. Themethod may be used to mount lenses, crystals, gratings, filters, fibersand sub-assemblies of the same, among others.

FIG. 11 illustrates the construction of a preferred sheet 78 which is abuilding block component in the method of the present invention tomanufacture optical assemblies. The sheet 78 includes an array ofmicro-frames 82 which are, preferably, photo-chemically etched orstamped into the sheet 78. Preferably, each frame 82 is supported by apair of tabs 80 which are also, preferably, photo-chemically etched orstamped into the sheet 78. Referring to FIG. 13, the tabs 80 arepreferably narrowed at the ends 81 to facilitate detachment of themicro-frames 82. Each frame 82 includes a substantially central cavity84. For a given sheet 78, it is preferred that all the cavities 84 aresimilarly shaped and dimensioned. Referring to FIGS. 12A and 12B,preferably, the micro-frames 82a include a generally square orrectangular shaped cavity 84a and the micro-frames 82b include agenerally circular cavity 84b, respectively. As discussed later herein,a combination of these preferred micro-frames 82a and 82b facilitate themethod of the present invention. Of course, the frames 82 may bealternately shaped, as required or desired.

Referring to FIG. 11, the required pattern of photo-chemical etching orstamping is dependent on the application which in turn determines themost suitable dimensional and material specifications of the sheet 78.Appropriate design rules and photo-chemical machining are available fromNewcut, Inc. of Newark, N.Y. Those skilled in the art will be aware thatthe sheet 78 can also be formed using alternative means, such asstamping, laser machining, ultrasonic machining and the like.

Preferably, and referring to FIG. 11, the sheets 78 are fabricated forma metallic material, and more preferably from stainless steel or copper.Of course, other metals, alloys, plastics and ceramics may be utilizedwith efficacy, as required or desired, to suit the needs of theparticular application. Once the required array has been patterned intothe sheets 78 (FIG. 11), the sheets 78 are preferably electro-platedwith an appropriate metal to the proper thickness for brazing. Ofcourse, the electrop-lating need only be applied to those surfaces ofthe sheets 78 which are to be brazed. Preferably, for stainless steelsheets 78 a 20 microinch (0.508 micrometer) copper plating is applied.Preferably, for copper sheets 78 a 20 microinch (0.508 micrometer)silver plating is applied. Electro-plating services are available fromany one of a number of sources well known to those skilled in the art.

Referring to FIGS. 14A and 14B, the electro-plated sheets 78 are alignedand brazed together to form a laminate unit (pallet) 90. The brazing isperformed in the appropriate atmosphere or vacuum with applied heat andforce. The sheets 78 may also be joined utilizing alternative means,such as welding, soldering and gluing among others, as required ordesired. As illustrated in FIGS. 15A to 15D, the alignment and brazingof the sheets 78 results in a layering of the arrays of micro-frames 82to form optics receiving micro-fixtures 86 each having a cavity 88.Preferably, the sheets 78 are layered such that one or more micro-frames82a are laminated onto one or more micro-frames 82b which together formthe fixture 86. This, advantageously, results in a seat 102 since thegenerally square cavity 84a (FIG. 12A) is dimensioned to be larger thanthe generally circular cavity 84b (FIG. 12B). In one preferred form ofthe present invention, a plastic is molded to form the laminate unit orpallet 90. The mold for such a manufacturing process can be madeutilizing a metal laminate pallet and an electroforming process tocreate a "reverse" master.

Referring to FIGS. 14B, 15B, 15C and 15D, an optical element 92 isaffixed in each cavity 88 of the array of fixtures 86 of the laminateunit (pallet) 90. As mentioned above, the optical element 92 can includelenses, crystals, gratings, filters, fibers and sub-assemblies of thesame, among others. Preferably, the optical elements 92 are attached tofixtures 92 using #118-03 B-stageable epoxy adhesive with 114-20 slowdrying thinner available from Creative Materials, Inc. of Tyngsboro,Mass. Typically, a pre-cure is performed to drive off any thinnerleaving a tacky surface on the fixture cavity 88. The optical element 92is inserted, seated on the seat 102, and if needed aligned in thefixture cavity 88. A final cure is performed, by reflowing theB-stageable epoxy so that the optical element 92 is securely attached tothe fixture 86, as shown in FIGS. 15C and 15D. Advantageously, the ports110 (FIG. 12A) of the generally square cavity 84a facilitate theapplication of epoxy and the square shape of the cavity 84a facilitatesalignment, if needed, of the optical element 92. Optionally, other typesof glues or other attachment means, such as pins, locks, clamps andsolders among others, may be used with efficacy to affix the opticalelement 92 to the fixture 86 giving due consideration to the desiredgoal of providing a reliable, clean and inert attachment.

Referring to FIGS. 16A, 16B and 16C the optics-loaded laminate units(pallets) 90 are aligned and, preferably, clamped to prepare for theirattachment. Of course, the laminate units 90 may be stacked in alternatemanners, as required or desired, to suit the needs of the particularapplication. Preferably, the laminate units (pallets) 90 are laserwelded to form a laminate stack 94 with an array of optical assemblies96. Preferably, and as shown in FIG. 16C, the fixtures are stacked suchthat the generally circular cavities 84b (FIG. 12B) are positioned ateither end of the resulting optical assembly 96. Preferably, the weldingutilizes a Nd:YAG industrial laser such as available from Unitek-MiyachiCorp. of Monrovia, Calif. Alternatively, the laminate units (pallets) 90may be attached to one another by resistance welding, soldering andgluing, for example, by using epoxy. In this manner, optical assemblies96 are created with each optical assembly 96 including one or moreoptics-loaded fixtures 86 attached to one another. Advantageously, allfunctional testing of the optical assemblies 96 can be performed in thisarray format while the assemblies 96 are still part of the laminatestack 94. Of course, the number of optical elements 92 comprising eachoptical assembly 96 is dependent on the particular application. This inturn determines the number of laminate units 90 (pallets) included in alaminate stack 94.

The optical assemblies 96 (FIGS. 16B and 16C) can then be removed fromthe laminate stack 94 by conventional trimming methods, such aspunching, shearing or stamping. Optionally, only a portion or portionsof the laminate stack 94 may be removed by punching, shearing orstamping, as required or desired. This permits additional elements to beadded to a selected portion or portions of the laminate stack 94.

In one preferred embodiment of the present method, a base 98 (FIG. 17)is used to house the optical assembly 96 (FIG. 16C). Referring to FIG.16B, one of the pair of set of tabs 80 associated with each opticalassembly 96 is removed by punching, shearing or stamping. The base 98 isgenerally yoke-shaped and has a cavity 100 which is configured anddimensioned to accommodate the fixtures 86 (see FIG. 15C) of the opticalassembly 96. Of course, the base 98 can be alternately configured asneeded or desired, giving due consideration to the goals of providing asupport for the optical assembly 96 and for permitting the assembly 96to be conveniently mounted in optical devices. Preferably, the base 98is fabricated from stainless steel though other alloys, metals, ceramicsand plastics may be utilized with efficacy. The base 98 may bemanufactured by machining, molding, forging or casting. In one preferredform of the present invention, the base 98 is molded from a hightemperature engineering plastic. Preferably, the base 98 is laser weldedto the optical assembly 96. Preferably, this welding utilizes a Nd:YAGindustrial laser as described above. Alternatively, the base 98 may beattached to the optical assembly 96 by resistance welding, soldering andgluing, for example, by using epoxy. In this manner, optical assemblies96 in combination with a base 98 are created. Advantageously, and asmentioned before, all functional testing and adjusting of the opticalassemblies 96 can be performed in this array format while the assemblies96 are still part of the laminate stack 94. Also, as described above,the optical assemblies 96 (FIGS. 16B and 16C) can then be removed fromthe laminate stack 94 by conventional trimming methods, such aspunching, shearing or stamping.

Referring to FIG. 11, the dimensions and configuration of the etchedsheet 78 are dictated by the particular application. In some cases, asheet 78 with a thickness of about 0.005 inches (0.127 mm) and agenerally square shape with an about 4 inches (0.1016 m) side ispreferred. The array size of the micro-frames 82 can be selected, asrequired or desired. In some cases, an array of frames 82 comprising 8rows with 16 frames 82 in each row is preferred. The dimensioning andconfiguration of the frames 82 is dependent on the particular opticalelements 92 (FIG. 15C) utilized. In one preferred form, the outerdiameter of the frames 82a (FIG. 12A) and 82b (FIG. 12B) is about 2.108mm (0.083 inches), the generally square cavity 84a (FIG. 12A) has sidesof about 1.422 mm (0.056 inches), and the generally circular cavity 84b(FIG. 12B) has a diameter of about 1.041 mm (0.041 inches). The numberof sheets 78 in each laminate unit 90 (pallet) is dependent on thethickness of each sheet 78 and that of the optical element 92.Similarly, the number of laminate units (pallets) 90 forming a laminatestack 94 is dictated by the particular application and hence thecomponents of the optical assembly 96.

FIGS. 12A and 12B, have illustrated etched micro-frames 82a and 82b,respectively, with a substantially circular outer perimeter. This formsa fixture 86 (FIG. 15B) with a substantially circular outer perimeter.This shape is generally preferred for coaxial type of opticalapplications, though other shapes may be utilized with efficacy, asneeded or desired. FIG. 18 illustrates a fixture 86' with a generallysquare or rectangular outer perimeter. This shape is preferred forplanar type of applications, wherein the fixtures 86' can be mounted ina slot (not shown) or against a stop (not shown). A platform (not shown)may be used to mount the fixtures 86', generally perpendicular to theplatform, to form a planar optical assembly.

Referring to FIGS. 19A and 19B, in one form of the present inventionframes 82b may be layered to form a fixture 86" having a substantiallyannular seat 104, by utilizing one or more frames 82 with a cavity 84bsmaller than the other cavities 84b. Such a configuration is preferredin the mounting of optical elements 92 which are generally spherical,such as ball lenses.

The method of the present invention is also well suited for theautomated manufacture of the temperature compensated optical isolator 10(shown, for example, in FIG. 5) of the present invention. Preferably,the polarizer 12 and the Faraday rotator 14 are mounted, preferablyusing the B-stageable epoxy, in the fixtures 86 (FIGS. 15B, 15C and 15D)to form arrays in the laminate units 90 (FIG. 14B). The thickness of thepolarizer 12 and the Faraday rotator 14, which can be dependent on thewavelength requirements of the application, will dictate the number ofsheets 78 (FIG. 11) needed to form a pair of respective first and secondlaminate units or pallets 90 (FIG. 14B). In some cases, up to two sheets78 and up to four sheets 78 (FIG. 11) of about 0.005 inches (0.127 mm)are sufficient to house a POLARCOR™ polarizer 12 about 200 μm thick anda Latching Garnet Faraday rotator 14 about 300 to 500 μm thick,respectively. The first and second laminate units or pallets 90 (FIG.14B) containing the respective arrays of polarizers 12 and Faradayrotators 14 are aligned and are, preferably, laser welded utilizing aNd:YAG industrial laser, as described above, to form the laminate stack94 (FIG. 16B). One of the pair of set of tabs 80 (FIG. 16B) associatedwith each polarizer-Faraday rotator optical sub-assembly 96 is removedby punching, shearing or stamping to provide clearance for the base 56(FIG. 5). Preferably, the base 56 (FIG. 5) is then aligned with thesub-assembly 96 (FIGS. 16B and 16C) and laser welded to it.

Referring to FIG. 5, the analyzer 16 is aligned with and adhered to thebimetallic element 46. Preferably, the B-stageable epoxy or solder isused to attach the analyzer 16 to the bimetallic element 46. Thebimetallic element 46 is attached to the base 56, and is preferablywelded to the base 56. This results in the creation of an array oftemperature compensated isolators 10 housed in the laminate stack 94(FIG. 16B). The isolators 10 can then be trimmed out of the array bypunching, shearing or stamping.

The efficiency and modularity of the method of the present invention islargely due to the lamination of arrays of micro-fixtures. It will beapparent to those skilled in the art that desirably the method of thepresent invention is well adapted to automated manufacturing of opticalassemblies or other micro-assemblies. The method can be used incombination with conventional pick-and-place type of robotics whichresults in high speed, high volume production, thereby desirablymaintaining low manufacturing costs. Advantageously, the method can betailored to assemble a wide variety of optical components and isadaptable to a wide range of applications.

While the components and method of the present invention have beendescribed with a certain degree of particularity, it is manifest thatmany changes may be made in the specific designs, constructions andmethodology hereinabove described without departing from the spirit andscope of this disclosure. It is understood that the invention is notlimited to the embodiments set forth herein for purposes ofexemplification, but is to be defined only by a fair reading of theappended claims, including the full range of equivalency to which eachelement thereof is entitled.

What is claimed is:
 1. A method of manufacturing optical assemblies,comprising the steps of:creating in each of a plurality of sheets anarray of frames; stacking said sheets to form an array of fixturescomprising two or more pallets; attaching an optical element in each ofsaid fixtures to form an array of optical elements in each said two ormore pallets; layering said two or more pallets to form a laminationsuch that said optical elements are substantially aligned to create anarray of said optical assemblies; and removing said optical assembliesfrom said lamination.
 2. The method of claim 1, wherein said step ofstacking includes the step of laminating said plurality of sheets toform said array of fixtures in said two or more pallets.
 3. The methodof claim 2, wherein before said step of laminating is included the stepof creating an array of frames in said plurality of sheets.
 4. Themethod of claim 3, wherein said step of creating includes the step ofphoto-chemically etching to form said array of frames in said pluralityof sheets.
 5. The method of claim 3, wherein said step of creatingincludes the step of stamping to form said array of frames in saidplurality of sheets.
 6. The method of claim 2, wherein said step oflaminating includes the step of brazing to form said array of fixturesin said two or more pallets.
 7. The method of claim 1, wherein said stepof attaching includes the step of gluing to form said array of opticalelements in each said two or more pallets.
 8. The method of claim 1,wherein said step of layering includes the step of welding to form saidlamination including said array of said optical assemblies.
 9. Themethod of claim 1, wherein said step of removing includes the step ofeither punching, shearing or stamping to form said optical assemblies.10. The method of claim 1, wherein between said steps of layering andremoving are included the steps of:detaching one or more tab membersconnected to said array of said optical assemblies; and affixing one ormore bases to support one or more of said optical assemblies.
 11. Themethod of claim 10, wherein said step of detaching includes the step ofeither punching, shearing or stamping.
 12. The method of claim 10,wherein said step of affixing includes the step of welding.
 13. Themethod of claim 10, wherein said one or more bases are generally yokeshaped.
 14. The method of claim 10, further including the step ofjoining an optical component to said one or more bases.
 15. A method ofmanufacturing a temperature compensated optical isolator, comprising thesteps of:forming an array of fixtures in a pair of first and secondpallets; attaching a polarizer in each said fixtures of said firstpallet to form an array of said polarizers; attaching a Faraday rotatorin each said fixtures of said second pallet to form an array of saidFaraday rotators; layering said first and second pallets to form alamination such that said array of said polarizers is substantiallyaligned with said array of said Faraday rotators to create an array ofoptical sub-assemblies; affixing a base to each one of said opticalsub-assemblies of said lamination; joining a bimetallic element coupledto an analyzer to each one of said bases to form an array of saidisolators; and removing said isolators from said lamination.
 16. Themethod of claim 15, wherein said step of forming includes the step oflaminating two or more sheets to form said array of fixtures in saidfirst pallet and said second pallet.
 17. The method of claim 16, whereinbefore said step of laminating is included the step of creating an arrayof frames in said two or more sheets.
 18. The method of claim 17,wherein said step of creating includes the step of photo-chemicallyetching to form said array of frames in said two or more sheets.
 19. Themethod of claim 17, wherein said step of creating includes the step ofstamping to form said array of frames in said two or more sheets. 20.The method of claim 16, wherein said step of laminating includes thestep of brazing to form said array of fixtures in said first pallet andsaid second pallet.
 21. The method of claim 15, wherein said steps ofattaching include the steps of gluing to form said array of polarizersand said array of Faraday rotators.
 22. The method of claim 15, whereinsaid step of layering includes the step of welding to form saidlamination including said array of optical sub-assemblies.
 23. Themethod of claim 15, wherein said step of affixing includes the step ofwelding.
 24. The method of claim 15, wherein said step of joiningincludes the step of welding.
 25. The method of claim 15, wherein saidstep of removing includes the step of either punching, shearing orstamping.
 26. The method of claim 15, wherein between said steps oflayering and affixing is included the step of detaching a plurality oftab members from said optical sub-assemblies.
 27. The method of claim26, wherein said step of detaching includes the step of either punching,shearing or stamping.
 28. A method of fabricating assemblies of smallcomponents, comprising the steps of:creating in each of a plurality ofsheets an array of frames; stacking said sheets to form a plurality ofpallets, each including an array of fixtures; affixing said componentsin said fixtures to form arrays of said components in said pallets;stacking said pallets to form a laminate stack such that said componentsare substantially aligned to create an array of said assemblies; andremoving said assemblies from said laminate stack.
 29. The method ofclaim 28, wherein said step of forming a plurality of pallets includesthe step of laminating a plurality of sheets.
 30. The method of claim28, wherein said step of forming a plurality of pallets includes thestep of molding.
 31. The method of claim 28, wherein said fixturescomprise micro-fixtures.
 32. A plurality of optical assemblies,comprising:a plurality of optical elements; and two or more palletsstacked together to form a plurality of fixtures, each fixture having anoptical element therein, wherein said respective pallets are formed bystacking a plurality of sheets, each sheet having a plurality of framestherein, said sheets with said frames being aligned to form saidfixtures and said pallets are stacked together such that said opticalelements therein form stacks of optical elements to form said pluralityof optical assemblies.